As an AC superconducting cable, a three-core-twisted-type cable is commonly known that is formed by twisting three cable cores together. FIG. 7 is a cross-sectional view of a three-core-twisted-type cable for the three-phase AC use. A superconducting cable 100 has a structure in which three cable cores 102 are twisted together and housed in a heat-insulated pipe 101. The heat-insulated pipe 101 has a dual-pipe structure composed of an outer pipe 101a and an inner pipe 101b, between which a heat-insulating material (not shown) is placed. The space between the outer pipe 101a and the inner pipe 101b is evacuated in vacuum. An anticorrosion covering 104 is provided on the outer circumference of the heat-insulated pipe 101. Each of the cable cores 102 comprises, from the center in the following order, a former 200, a superconducting conductor layer 201, an insulating layer 202, a superconducting shielding layer 203, and a protecting layer 204. A space 103 enclosed by the inner pipe 101b and the cable cores 102 forms a channel for a coolant such as liquid nitrogen.
When AC transmission is performed by using the above-described superconducting cable, not only is AC loss caused due to the inductance but also current at the time of the short circuiting is large, so that the temperature may rise excessively due to the loss at that moment. In contrast to the AC transmission, DC transmission using a superconducting cable not only eliminates the AC loss but also decreases the short circuit current. As a DC superconducting cable, Patent literature 1 has proposed a superconducting cable formed by twisting together three cable cores, each having a superconducting conductor and an insulating layer. In this superconducting cable, each of the cores comprises a superconducting conductor, an insulating layer provided on the outer circumference of the conductor, and a return conductor that is composed of superconducting wires provided on the outer circumference of the insulating layer. Unipolar transmission is performed by using the superconducting conductors as the outward line and the return conductors as a return line.
Patent literature 1: the published Japanese patent application Tokukai 2003-249130.